1. Field of the Invention
The present invention relates to a semiconductor-device copper-alloy bonding wire which connects an electrode on a semiconductor device with the wiring of a circuit-wiring substrate.
2. Description of the Related Art
Presently, a thin line (bonding wire) having a diameter of 20 to 100 μm or so is mainly used as a bonding wire which connects an electrode on a semiconductor device and an external terminal together. A supersonic and thermal pressure-bonding combination scheme is generally applied to join a bonding wire, and for example, a general-purpose bonding device, a capillary jig, which causes a bonding wire to pass through the interior of the general-purpose bonding device and is used for connection, are used. The leading end of a bonding wire is heated and melted by arc heat input, a ball is formed by surface tension and is subjected to pressure-bonding and joining onto an electrode of a semiconductor device heated at a range from 150 to 300° C., and then the bonding wire is subjected to wedge joining directly together with a lead.
Generally, high purity 4N group gold (purity >99.99 mass %) is used as the material of a bonding wire so far. However, because gold is expensive and a power IC needs a thicker wire (diameter: 50 to 100 μm or so), a bonding wire of another kind of metal having a cheaper material cost is desired.
From the standpoint of the wire bonding technology, it is desirable to form a ball having a good sphericity in forming the ball, and to form the joining portion of the ball and an electrode in a complete circular shape as much as possible, and it is necessary to obtain a sufficient joining strength. To cope with lowering a joining temperature, thinning a bonding wire, and the like, successive bonding without causing peeling or the like, and a sufficient joining strength are required in case of performing wedge connection on a bonding wire on a lead terminal or a wiring substrate.
In a resin sealing step of filling a thermoset epoxy resin having a high viscosity, there is a problem such that a bonding wire may be deformed and contact an adjoining wire. What is more, since the wire-bonding pitch becomes narrow, the length of a wire becomes long, and a wire bonding itself becomes thin, it is necessary to prevent deformation of a wire in resin filling as much as possible. Because of the improvement of the strength of a wire, such a deformation is controllable on some level, but without overcoming problems such that loop control becomes difficult and the strength in joining is reduced, practical use is hardly realized.
It is desirable to obtain comprehensive wire characteristics such as facilitating loop control in a bonding step, improving joining of a bonding wire with an electrode or a lead, and suppressing excessive deformation of a bonding wire in a resin filling step after bonding to satisfy the foregoing requests.
Unexamined Japanese Patent Applications No. S61-251062, No. S62-78861, No. S61-20693, and No. S62-78862 disclose copper bonding wires which have an inexpensive material cost, a good electrical conductivity, and which facilitate joining and loop formation. According to the copper bonding wire, however, because a ball portion thereof has a higher hardness than Au, it may cause a damage like a crack to a chip when the ball is deformed on a pad electrode to join the ball with the electrode. Regarding wedge joining of a copper bonding wire, because the production margin is narrow in comparison with Au, reduction of mass productivity is concerned. Further, because of chemical compound growth at a joined portion, reduction of long-term reliability is also concerned. Those are the reasons why the practical application of a copper bonding wire is not in progress.
A problem from the standpoint of practical application in using a copper bonding wire is that a chip may be damaged in joining because a ball is hard. As a solution against such a problem, Japanese Unexamined Patent Application No. S61-251062 discloses a scheme of suppressing the contained amount of impurities to less than or equal to 10 ppm, and Japanese Unexamined Patent Application No. S62-78861 discloses a scheme of suppressing the contained amounts of hydrogen and oxygen in a highly-pure copper to less than or equal to specific values. Even though such schemes can prevent a ball from being hardened, the ball may have a deformation problem, such as a petal shape, an irregular shape, or an eccentric shape if the bonding wire is merely high purified. The shape of a ball in joining and chip damaging are in a contradictory relationship, and overcoming both becomes a problem to be solved. Merely causing a bonding wire to be highly purified leaves many problems such that the strength is reduced and complex loop control becomes difficult, the recrystallized grain of a neck portion becomes coarse and the loop height cannot be uniformed, and the pull strength becomes weak.
Adding alloy element to a copper bonding wire which enhances ball joining has been reported. For example, Japanese Unexamined Patent Application No. S61-20693 discloses a bonding wire which contains Mg, Ca, rare-earth element, Ti, Hf, V, Nb, Ta, Ni, Pd, Pt, Au, Cd, B, Al, In, Si, Ge, Pb, P, Sb, Bi, Se and Te at a mass % of 0.001 to 2, and a remaining element thereof is substantially copper for the purpose of improving the joining strength. However, merely adjusting alloying elements does not cause the improvements of individual characteristics, such as the joining characteristic, and a loop shape, so that improvement on the comprehensive usability is hardly achieved. Further, ones of the reasons why the practical application of a copper bonding wire is not advanced well are practical problems such that even though improvement by adding metal elements is achieved, the characteristics varies widely wire by wire or product lot by production lot, and a change in a characteristic or the like originating from a period how long the product is left before it is used is not expectable.
Copper is likely oxidized in the air, and formation of a fine oxide film of copper brings about a concern such that the ball joining characteristic and the wedge joining characteristic may be deteriorated. If oxygen is contained in copper at a high concentration (density), some negative effects may be concerned, but the correlation with joining is not always apparent. There is a possibility that wedge joining of a bonding wire just after production is improved by simply thinning a copper oxide film, reducing the concentration of solution oxygen, but it is difficult to precisely manage such an improvement, and normally, an oxide film grows while a wire product is stocked. This brings about a problem of deteriorating a characteristic. To delay oxidation of the surface of a copper bonding wire, applying a rust-preventive agent has been proposed, and is disclosed by, for example, Japanese Unexamined Patent Application No. S62-78862. The rust-preventive agent is effective for oxidation suppression, but the agent itself disturbs joining. It is difficult to improve the shape of a ball in joining and the joining strength by merely applying the rust-preventive agent, so that the application of the rust-preventive agent has a limitation.
It is an object of the invention to provide a semiconductor-device copper-alloy bonding wire which overcomes the foregoing problems of the conventional technologies, improves the shape of a ball portion in joining, mainly consists of copper, and is inexpensive in comparison with a gold bonding wire.